Illumination portion for an adaptable resolution optical encoder

ABSTRACT

An illumination portion of an optical encoder comprising a scale track extending along a measuring axis direction, an imaging portion, and a detector configuration. The illumination portion comprises: a light source configured to output source light; a collimation portion; and a structured illumination generating portion comprising a beam-separating portion and an illumination grating and configured to input the source light and output structured illumination to the scale track. The beam-separating portion is arranged to input the source light and output a first source light portion and a second source light portion to the illumination grating, such that they form beams that are spaced apart from one another along the measuring axis direction. The illumination grating is configured to diffract the first and second source light portions to the scale track such that only two orders of diffracted light overlap within an imaged region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/730,790, filed Dec. 28, 2012, which is acontinuation-in-part of U.S. patent application Ser. No. 13/717,586,filed Dec. 17, 2012, which claims the benefit of U.S. ProvisionalApplication No. 61/580,133, filed Dec. 23, 2011, the disclosures ofwhich are incorporated by reference herein in their entirety.

FIELD

The present application relates generally to precision measurementinstruments and more particularly to optical displacement encoders.

BACKGROUND

Various optical displacement encoders are known that use a readheadhaving an optical arrangement that images a scale pattern to aphotodetector arrangement in the readhead. The image of the scalepattern displaces in tandem with the scale member, and the movement orposition of the displaced scale pattern image is detected with aphotodetector arrangement. Conventional imaging, self-imaging (alsocalled Talbot imaging), and/or shadow imaging may be used to provide thescale pattern image in various configurations.

Optical encoders may utilize incremental or absolute position scalestructures. An incremental position scale structure allows thedisplacement of a readhead relative to a scale to be determined byaccumulating incremental units of displacement, starting from an initialpoint along the scale. Such encoders are suitable for certainapplications, particularly those where line power is available. However,in low power consumption applications (e.g., battery powered gauges andthe like), it is more desirable to use absolute position scalestructures. Absolute position scale structures provide a unique outputsignal, or combination of signals, at each position along a scale. Theydo not require continuous accumulation of incremental displacements inorder to identify a position. Thus, absolute position scale structuresallow various power conservation schemes. A variety of absolute positionencoders are known, using various optical, capacitive, or inductivesensing technologies. U.S. Pat. Nos. 3,882,482; 5,965,879; 5,279,044;5,886,519; 5,237,391; 5,442,166; 4,964,727; 4,414,754; 4,109,389;5,773,820; and 5,010,655 disclose various encoder configurations and/orsignal processing techniques relevant to absolute position encoders, andare hereby incorporated herein by reference in their entirety.

One type of configuration that is utilized in some optical encoders is atelecentric arrangement. U.S. Pat. Nos. 7,186,969; 7,307,789; and7,435,945, each of which is hereby incorporated herein by reference inits entirety, disclose various encoder configurations that utilizeeither singly or doubly telecentric imaging systems for imaging theperiodic pattern of light and sensing displacement of the periodic scalestructure. Telecentric imaging systems provide certain desirablefeatures in such optical encoders.

One issue with regard to the design of such optical encoders is thatusers generally prefer that the readheads and scales of the encoders beas compact as possible. A compact encoder is more convenient to installin a variety of applications. For certain precision measurementapplications, high resolution is also required. However, the prior artfails to teach configurations that provide certain combinations of highresolution, range-to-resolution ratio, robustness, compact size, anddesign features that allow a number of encoder resolutions to beprovided using shared manufacturing techniques and components, and thatfacilitate low cost as desired by users of encoders. Improvedconfigurations of encoders that provide such combinations would bedesirable.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

The principles disclosed herein are directed to improved opticaldisplacement encoder configurations that provide improved combinationsof high resolution, range-to-resolution ratio, compact size, robustness,and that allow a number of encoder resolutions to be provided usingshared manufacturing techniques and components.

The disclosure of U.S. patent application Ser. No. 13/717,586 includedthe use of a phase grating in an illumination portion to substantiallyblock zero order light from reaching an encoder scale grating in orderto diminish related errors in the encoder displacement signals, andother encoder configurations have utilized a blocking portion in anaperture of an imaging configuration to substantially block unwantedzero order light from reaching the detector. However, the inventor(s)have determined that these methods still allow residual zero order light(e.g., on the order of a few percent of the signal light, or less) toproduce unwanted periodic errors in the spatially modulated image lightprovided at the detector configuration. Therefore, further improvementsin diminishing zero order light propagation remain desirable in certainencoder configurations.

In various embodiments disclosed herein, an illumination portion of anoptical encoder that comprises a scale track extending along a measuringaxis direction, an imaging portion, and a detector configuration isdisclosed. The illumination portion comprises a light source configuredto output source light having a wavelength λ and a structuredillumination generating portion configured to input the source light andprovide structured illumination, wherein the structured illuminationcomprises an illumination fringe pattern oriented transversely to themeasuring axis direction which is input to the scale track. The scaletrack is configured to spatially modulate the input illumination fringepattern and output scale light comprising spatially modulated imagelight. The detector configuration and imaging configuration areconfigured such that only scale light arising from an imaged region ofthe scale track is imaged to the detector configuration. The structuredillumination generating portion comprises a beam separating portion andan illumination grating. The beam separating portion is arranged toinput the source light and is configured to output a first source lightportion and a second source light portion to the illumination grating,such that the first and second source light portions form beams that arespaced apart from one another along the measuring axis direction at aplane proximate to the illumination grating. The illumination grating isconfigured to diffract the first and second source light portions acrossan operating gap to the scale track such that only two orders ofdiffracted light overlap within the imaged region at a plane coincidingwith the scale track, and provide the illumination fringe pattern in theimaged region.

In some embodiments, the imaged region may have a dimension D along themeasuring axis direction, the first source light portion and the secondsource light portion may be spaced apart from one another along themeasuring axis direction at the plane proximate to the illuminationgrating by a separation distance B, and the separation distance B may beequal to or greater than the imaged region dimension D. In someembodiments, the first source light portion and the second collimatedsource light portion may each have a dimension along the measuring axisdirection, at the plane proximate to the illumination grating, that isgreater than the imaged region dimension D.

In some embodiments, the illumination portion may further comprise acollimation portion arranged to collimate the source light that is inputto the beam separating portion.

In some embodiments, the illumination portion may further comprise acollimation portion arranged to collimate the first source light portionand second source light portion that are output to the illuminationgrating.

In some embodiments, the two orders of diffracted light that overlapwithin the imaged region at a plane coinciding with the scale track maybe a + first order diffracted light from the first source light portionand a − first order diffracted light from the second source lightportion.

In some embodiments, the beam separating portion may further comprise afirst beam splitting element surface and a reflective surface. The beamsplitting element surface may be configured to receive light from thesource light source and output the first and second source lightportions along a first beam path and transmit the second source lightportion toward the reflective surface; and the reflective surface may beconfigured to reflect the second source light portion direction of thesecond illumination light portion toward the scale track along a secondbeam path that is spaced apart from the first beam path. In someembodiments, the beam splitting surface and the reflective surface maybe parallel. In some embodiments, the beam splitting surface and thereflective surface may be surfaces of separate elements. In someembodiments, the beam splitting surface and the reflective surface maybe surfaces of the same beam splitting element. In some embodiments, thebeam splitting element may be a shearing plate. In some embodiments, thefirst and second beam paths may be approximately perpendicular to theillumination grating.

It will be appreciated that while the methods are primarily describedherein in terms of imaging and spatial filtering, such concepts may alsobe described in terms of the light ray components of the optical system,including diffracted rays, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages will becomemore readily appreciated as the same become better understood byreference to the following detailed description, when taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a partially schematic exploded diagram of an encoderconfiguration with a doubly telecentric arrangement and a scale withabsolute, reference, and incremental track patterns, and utilizingconventional imaging techniques;

FIGS. 2A-2C are diagrams of the incremental scale track pattern, imageintensity, and detector arrangement of the encoder configuration of FIG.1;

FIG. 3 is a partially schematic exploded diagram of an encoderconfiguration with a doubly telecentric arrangement and a scale withabsolute, reference, and incremental track patterns, and utilizingspatial filtering and imaging principles in accordance with principlesdisclosed herein;

FIGS. 4A-4D are diagrams of the illumination fringe pattern, incrementalscale track pattern, resulting moiré image intensity, and detectorarrangement of the encoder configuration of FIG. 3;

FIG. 5 is a chart illustrating modulation transfer functionscorresponding to various sets of design parameters;

FIG. 6 is a chart illustrating the dependence of depth of field (% DOF),spatial harmonic content, and optical signal power on the dimension ofan aperture along the measuring axis direction;

FIG. 7 is a partially schematic exploded diagram of one exemplaryembodiment of the encoder configuration of FIG. 1;

FIG. 8 is a partially schematic exploded diagram of one exemplaryembodiment of the encoder configuration of FIG. 3;

FIG. 9 is a diagram of an alternative configuration of the phase gratingportion of the embodiment of FIG. 8;

FIGS. 10A and 10B are diagrams of the scale track pattern arrangementsof the encoder configurations of FIGS. 1 and 3, respectively;

FIG. 11 is a table illustrating parameters for various scale anddetector track combinations for the encoder configuration of FIG. 3;

FIG. 12 shows a schematic cross-sectional diagram showing differingoptical paths through a doubly telecentric imaging encoder arrangement;

FIGS. 13A and 13B show a configuration that is another exemplaryembodiment of a practical implementation of an encoder configurationaccording to principles disclosed herein;

FIG. 14 shows an analysis of the configuration shown in FIG. 13,indicating how a phase grating provides the operational diffractionorders that provide the optical intensity signals on the detector;

FIG. 15 is a partially schematic exploded diagram of an encoderconfiguration which includes a first alternative embodiment of anillumination portion;

FIG. 16 is a schematic drawing of an encoder configuration whichincludes a second alternative embodiment of an illumination portionwhich includes elements in addition to those shown in FIG. 15, and whichmay be used in a reflective encoder configuration;

FIG. 17 is a drawing of an illumination portion which may be used in anencoder configuration according to the principles disclosed herein;

FIG. 18 is a drawing of an illumination portion which may be used in anencoder configuration according to the principles disclosed herein;

FIG. 19 is a drawing of an illumination portion which may be used in anencoder configuration according to the principles disclosed herein;

FIG. 20 is a drawing of an illumination portion which may be used in anencoder configuration according to the principles disclosed herein;

FIG. 21 is a drawing of an illumination portion which may be used in anencoder configuration according to the principles disclosed herein; and

FIG. 22 is a drawing of an illumination portion which may be used in anencoder configuration according to the principles disclosed herein.

DETAILED DESCRIPTION

FIG. 1 is a partially schematic exploded diagram of an opticaldisplacement encoder configuration 100 with a doubly telecentricarrangement and a scale with absolute, reference, and incremental trackpatterns and utilizing conventional imaging techniques. Certain aspectsof the encoder configuration 100 are similar to encoder configurationsdescribed in copending and commonly assigned U.S. patent applicationSer. No. 12/535,561, filed Aug. 4, 2009, and U.S. patent applicationSer. No. 12/273,400, filed Nov. 18, 2008 (hereinafter the '400application), which are hereby incorporated by reference in theirentireties. While the encoder configuration 100 is able to operateaccurately and effectively with an incremental scale track with arelatively coarse pitch (e.g., 20 microns), as will be described in moredetail below with reference to FIG. 3, the methods disclosed herein maybe utilized to allow an incremental scale track with a much finer pitch(e.g., 4 microns) to be utilized in a similar configuration.

As shown in FIG. 1, the encoder configuration 100 includes a scaleelement 110, a lens 140 for directing visible or invisible wavelengthsof light from a light source (not shown), and a doubly telecentricimaging configuration 180. The doubly telecentric imaging configuration180 comprises a first lens 181 at a first lens plane FLP, an aperture182 in an aperture component 182′ at an aperture plane AP, a second lens183 at a second lens plane SLP, and detector electronics 120 at adetector plane DP. In at least one embodiment, the scale element 110 isseparated from the first lens plane FLP by a distance d₀, the first lensplane FLP is separated from the aperture plane AP by a focal distance f,the aperture plane AP is separated from the second lens plane SLP by afocal distance f′, and the second lens plane SLP is separated from thedetector plane DP by a distance d₀′. The detector electronics 120 may beconnected to signal-generating and processing circuitry 190. The lightsource may also be connected to the signal-generating and processingcircuitry 190 by power and signal connections (not shown).

In the embodiment shown in FIG. 1, the scale element 110 includes ascale pattern 115 that comprises three scale track patterns: an absolutescale track pattern TABS1, a reference scale track pattern TREF1, and anincremental scale track pattern TINC1. The track pattern TABS1 isreferred to as an absolute scale track pattern because it providessignals usable to determine an absolute position over an absolutemeasuring range. In at least one embodiment, any conventional absolutescale pattern may be utilized for the absolute scale track patternTABS1. In at least one embodiment, the absolute scale track patternTABS1 may have a very “coarse” ABS resolution, on the order of thedetector dimension along the X axis.

For the incremental scale track pattern TINC1, in at least oneembodiment the incremental pitch may be relatively coarse (e.g., 20microns). As will be described in more detail below with respect to FIG.3, a finer pitch (e.g., 4 microns) may be made to be operable in asimilarly sized encoder configuration by utilizing the methods disclosedherein. The reference scale track pattern TREF1 is formed so that it canbe resolved to a level that allows it to indicate a particularincremental wavelength, so that the incremental wavelength (e.g., fromthe incremental scale track pattern TINC1) is not ambiguous relative toan absolute mark (e.g., from the absolute scale track pattern TABS1). Aswill be described in more detail below with respect to FIG. 10A, in atleast one embodiment, the reference scale track pattern TREF1 maycomprise a series of reference marks. In at least one embodiment, thereference marks may be formed as a series of barker patterns, which mayalso function as Vernier reference marks, and which may be formedaccording to a variety of known techniques.

FIG. 1 shows orthogonal X, Y, and Z directions, according to aconvention used herein. The X and Y directions are parallel to the planeof the scale pattern 115, with the X direction parallel to the intendedmeasuring axis direction MA 82 (e.g., perpendicular to elongated patternelements that may be included in the incremental scale track patternTINC1). The Z direction is normal to the plane of the scale pattern 115.

The detector electronics 120 include a detector configuration 125comprising three detector tracks DETABS1, DETREF1. and DETINC1, whichare arranged to receive light from the three scale track patterns TABS1,TREF1, and TINC1, respectively. The detector electronics 120 may alsoinclude signal processing circuitry 126 (e.g., signal offset and/or gainadjustments, signal-amplifying and combining circuits, etc.). In atleast one embodiment, the detector electronics 120 may be fabricated asa single CMOS IC.

In operation, as exemplified by the image channel for the incrementalscale track pattern TINC1, the light from the illumination source isdirected by the lens 140 to illuminate the incremental scale trackpattern TINC1 with source light 131. In some embodiments, the sourcelight 131 is coherent light. The incremental scale track pattern TINC1then outputs scale light 132. It will be appreciated that the limitingaperture 182, which has an aperture width AW along the X direction, actsas a spatial filter (as will be described in more detail below withrespect to FIG. 2) to select or limit the light rays that pass throughthe image channel for the incremental scale track pattern TINC1. FIG. 1illustrates three such light rays, two extreme rays and one central ray.As shown in FIG. 1, the lens 181 transmits the light rays towards thelimiting aperture 182. The limiting aperture 182 transmits the rays asspatially filtered image light 133 to the second lens 183, and thesecond lens 183 transmits and focuses the spatially filtered image lightto form an image of the scale track pattern TINC1 at the detector trackDETINC1.

Thus, when the incremental scale track pattern TINC1 is illuminated, itoutputs a track-specific spatially modulated light pattern to thedetector track DETINC1 of the detector electronics 120. An image of thespatially modulated light pattern is formed at an image plane IMGP,which may be made to be coplanar with the detector track DETINC1 (theimage plane IMGP being shown separately in FIG. 1 for purposes ofillustration). As shown in the image plane IMGP, the pattern of thescale image SI has a modulated scale image pitch P_(SI), which, in onespecific example embodiment, may be relatively coarse (e.g., 20microns).

Similar to the imaging of the spatially modulated light pattern from theincremental scale track pattern TINC1 on the detector track DETINC1,when the scale track patterns TREF1 and TABS1 are illuminated by thelight from the lens 140, they output track-specific spatially modulatedlight patterns (e.g., patterned light corresponding to their patterns)to the track-specific detector tracks DETREF1 and DETABS1, respectively,of the detector electronics 120. As noted above, the reference scaletrack pattern TREF1 (e.g., with barker patterns) indicates a particularincremental wavelength, so that the wavelength from the incrementalscale track pattern TINC1 is not ambiguous relative to the absolute markfrom the absolute scale track pattern TABS1. It will be appreciated thatall of the spatially modulated light patterns move in tandem with thescale 110.

As will be described in more detail below with respect to FIG. 11, ineach of the detector tracks DETINC1, DETABS1, and DETREF1, individualphotodetector areas are arranged to spatially filter their respectivelyreceived spatially modulated light patterns to provide desirableposition-indicating signals (e.g., the incremental detector trackDETINC1 producing quadrature signals, or other periodic signals having aspatial phase relationship that is conducive to signal interpolation).In some embodiments, rather than individual photodetector areas, aspatial filter mask with individual apertures may mask relatively largerphotodetectors to provide light-receiving areas analogous to theindividual photodetector areas, to provide a similar overall signaleffect according to known techniques.

In various applications, the detector electronics and light source aremounted in a fixed relationship relative to one another, e.g., in areadhead or gauge housing (not shown), and are guided along themeasuring axis relative to the scale 110 by a bearing system, accordingto known techniques. The scale may be attached to a moving stage, or agauge spindle, or the like, in various applications. It will beappreciated that the configuration shown in FIG. 1 is a transmissiveconfiguration. That is, the scale pattern 115 comprises light blockingportions and light transmitting portions (e.g., fabricated on atransparent substrate using known thin-film patterning techniques, orthe like) that output the spatially modulated light patterns to thedetector tracks by transmission. It will be appreciated that similarcomponents may be arranged in reflective embodiments, wherein the lightsource and the detector electronics are arranged on the same side of thescale 110, and positioned for angled illumination and reflection ifnecessary, according to known techniques.

In either transmissive or reflective scale patterns, the portions of thescale pattern that provide the light that is detected by the detectortracks (e.g., DETABS1, DETREF1, or DETINC1) may be referred to as thesignal-producing portions of the scale pattern, and it will beunderstood that other portions of the scale pattern generally provide aslittle light as possible to the detector tracks and may be referred toas signal-diminishing portions. It will be appreciated that thesignal-producing portions or the signal-diminishing portions of thescale pattern may be patterned according to the teachings herein, invarious embodiments. Stated another way, scale patterns which are“negatives” of each other may both produce useable signals, with theresulting signal variations also being approximately the “negative” ofeach other for a given reflective or transmissive arrangement. Thus, thescale patterns may be described in terms of “signal-varying portions,”and it will be understood that in various embodiments, thesignal-varying portions may comprise either the signal-producingportions or the signal-diminishing portions of the scale pattern.

FIGS. 2A-2C illustrate various aspects related to the optical signalchannel corresponding to the incremental scale track pattern TINC1 ofFIG. 1. More specifically, FIG. 2A illustrates the incremental scaletrack pattern TINC1 which has a scale pitch P_(SL). FIG. 2B is a graphof the resulting image intensity signal IMG1 from the light from theincremental scale track TINC1 at the detector plane DP. As shown in FIG.2B, the resulting image intensity has been spatially filtered (e.g., bythe aperture 182) so as to produce an approximately sinusoidal signal(e.g., as opposed to a square wave signal, such as would be producedfrom a non-filtered signal from the incremental scale track patternTINC1) and has a signal period P_(ISC). FIG. 2C is a diagram of theincremental detector track DETINC1 which, for purposes of illustration,has an image of the image intensity signal IMG1 from FIG. 2Bsuperimposed upon it. As shown in FIG. 2C, the detector track DETINC1 isconnected so as to output quadrature signals, with four detectorelements being within one period of the detector track wavelength λ_(d),which also corresponds to one period P_(ISC) of the image intensitysignal IMG1.

FIG. 3 is a partially schematic exploded diagram of an encoderconfiguration 300 with a doubly telecentric arrangement and a scale withabsolute, reference, and incremental track patterns, and utilizingspatial filtering and imaging techniques in accordance with theprinciples disclosed herein. Certain of the components and operatingprinciples of the encoder configuration 300 are approximately similar tothose of the encoder configuration 100 of FIG. 1, and may generally beunderstood by analogy. For example, 3XX series numbers in FIG. 3 thathave the same “XX” suffix as 1XX series numbers in FIG. 1 may designatesimilar or identical elements that may function similarly, except asotherwise described or implied below.

As shown in FIG. 3, the encoder configuration 300 includes a scaleelement 310, an illumination system or portion 360, and a doublytelecentric imaging configuration 380. The illumination system orportion 360 includes a light source 330 (e.g., an LED) for emittingvisible or invisible wavelengths of light, a lens 340, and a phasegrating 350. As will be described in more detail below, the phasegrating 350 may, in at least one embodiment, be utilized for producingstructured light patterns and may be located within the optical signalpath channels for the incremental and reference scale track patternsTINC2 and TREF2, but not the absolute scale track pattern TABS2. Thedoubly telecentric imaging configuration 380 comprises a first lens 381at a first lens plane FLP, an aperture 382 in an aperture component 382′at an aperture plane AP, a second lens 383 at a second lens plane SLP,and detector electronics 320 at a detector plane DP. The detectorelectronics 320 may be connected to signal-generating and processingcircuitry 390. The light source 330 may also be connected to thesignal-generating and processing circuitry 390 by power and signalconnections (not shown).

In the embodiment shown in FIG. 3, the scale element 310 includes ascale pattern 315 that comprises three scale track patterns: an absolutescale track pattern TABS2, a reference scale track pattern TREF2, and anincremental scale track pattern TINC2. In at least one embodiment, aconventional absolute scale track pattern may be utilized for theabsolute scale track pattern TABS2. In at least one embodiment, theabsolute scale track pattern TABS2 may have a relatively “coarse” ABSresolution, on the order of the detector dimension along the X axis.

As will be described in more detail below, the encoder configuration 300is designed to utilize certain spatial filtering and imaging principlesthat allow a fine pitch scale to provide larger pitch fringes thatcorrespond to the detector element pitch of an economical detector thatsenses the scale displacement. In order to produce the desired fringes,the phase grating 350 is designed to have a pitch that is close to thepitch of the incremental scale track pattern TINC2 and the referencescale track pattern TREF2 (e.g., a phase grating pitch of 5 microns ascompared to an incremental scale track pitch of 4 microns, and areference scale track pitch of 4.1 microns). The resulting fringe periodfrom the phase grating 350 and incremental scale track pattern TINC2 maybe relatively coarse (e.g., 20 microns) and may be slightly differentthan the fringe period produced by the phase grating 350 and thereference scale track pattern TREF2 (e.g., 22.77 microns).

As will be described in more detail below, the detected pattern isimaged with spatial filtering by the double telecentric imagingconfiguration 380, including the aperture 382 that blurs out or removesthe high spatial frequencies corresponding to the incremental andreference scale track patterns TINC2 and TREF2. In certainimplementations, the parameters are chosen so that the resultingmodulated image pitch of the spatially filtered pattern matches thepitch of a predetermined given detector (e.g., a detector designed for a20 micron incremental scale track pitch). Suitable aperture dimensionsmay be chosen to achieve the desired effect of the spatial filteringthat removes the high spatial frequencies and results in the desiredpattern fringe period. Certain teachings regarding such aperturedimensions for achieving desired spatial wavelength filtering aredescribed in more detail in commonly assigned U.S. Pat. No. 7,186,969,which is hereby incorporated by reference in its entirety.

As will be described in more detail below with reference to FIG. 10B, inat least one embodiment, the reference scale track pattern TREF2 mayinclude a series of reference marks which may be formed as barkerpatterns. The reference marks may also serve as Vernier reference marks.The reference scale track pattern TREF2 is designed so that it can beresolved to a level that allows it to indicate a particular incrementalwavelength for the incremental scale track pattern TINC2, so that theincremental wavelengths are not ambiguous relative to an absolute markfrom the absolute scale track pattern TABS2. In at least one embodiment,the combination of the reference track pattern TREF2 (e.g., a barkerpattern) and the incremental track pattern TINC2 may create a syntheticwavelength whose measured synthetic phase points to the correctincremental scale track pattern cycle (e.g., a measured synthetic phaseof zero may indicate a correct incremental cycle corresponding to thatphase).

As a specific example, the reference scale track pattern TREF2 may havea slightly different pitch (e.g., 4.1 microns producing a modulated andspatially filtered fringe pattern with a period of 22.77 microns) ascompared to the pitch of the incremental scale track pattern TINC2(e.g., 4.0 microns producing a modulated and spatially filtered fringepattern with a period of 20 microns) so that the reference scale trackpattern phase matches the incremental scale track pattern phase only atone specific point along a specified length (e.g., only matches at onepoint along a barker pattern length within the reference scale trackpattern). The position where the phases match defines a particularincremental wavelength for the incremental scale track pattern TINC2.

In one specific example embodiment, in the reference scale track patternTREF2, barker patterns may be provided at selected intervals (e.g., 0.6millimeters). The phase of each barker pattern (e.g., at the center ofthe pattern) matches (or has a constant phase offset from) the phase ofthe incremental scale track pattern TINC1 at the locations that are thespecified distance (e.g., 0.6 millimeters) apart. The syntheticwavelength of the incremental scale track pattern TINC2 and thereference scale track pattern TREF2 is larger than the barker patternlength. In at least one embodiment, this relationship may be expressedby stating that the synthetic wavelength of the incremental scale trackpattern and the reference (e.g., barker) scale track pattern is largerthan the barker pattern length L, such that L<pp′/(p′−p), where p is thepitch of the incremental scale track pattern TINC2, and p′ is the pitchof the barker pattern in the reference scale track pattern TREF2.

As shown in FIG. 3, the detector electronics 320 includes a detectorconfiguration 325 comprising three detector tracks, DETABS2, DETREF2,and DETINC2, arranged to receive light from the three scale trackpatterns, TABS2, TREF2, and TINC2, respectively. The detectorelectronics 320 may also include signal processing circuitry 326 (e.g.,signal offset and/or gain adjustments, signal-amplifying and combiningcircuits, etc.). In at least one embodiment, the detector electronics320 may be fabricated as a single CMOS IC.

In operation, light 331 (e.g., primary light) emitted from the lightsource 330 may be partially or fully collimated by the lens 340 over abeam area sufficient to illuminate the three scale track patterns TABS2,TREF2, and TINC2. The phase grating 350 is sized to diffract the sourcelight to provide diffracted structured light 331′ to the reference andincremental scale track patterns TREF2 and TINC2 (but not the absolutescale track pattern TABS2), in order to achieve the modulated andspatially filtered imaging effects described above. Then, as exemplifiedby the image channel for the incremental scale track pattern TINC2, theincremental scale track pattern TINC2 provides scale light 332 to thelens 381. It will be appreciated that the limiting aperture 382, whichhas an aperture width AW along the X-axis direction, acts as a spatialfilter (as will be described in more detail below with respect to FIGS.4 and 12) to select or limit the light rays that pass through the imagechannels. FIG. 3 illustrates three such light rays, two extreme rays andone central ray. As shown in FIG. 3, the lens 381 transmits the lightrays towards the limiting aperture 382. The limiting aperture 382transmits the rays as spatially filtered image light 333 to the secondlens 383, and the second lens 383 transmits and focuses the spatiallyfiltered image light to form a spatially modulated light pattern at thedetector track DETINC2. As noted above, and as will be described in moredetail below with respect to FIG. 4, in accordance with the principlesdisclosed herein the spatially modulated light pattern at the detectortrack DETINC comprises a modulated and spatially filtered fringepattern.

Similarly, when the scale track patterns TREF2 and TABS2 areilluminated, they output track-specific spatially modulated lightpatterns (e.g., patterned light corresponding to their patterns) to thetrack-specific detector tracks DETREF2 and DETABS2, respectively, of thedetector electronics 320. As noted above, the spatially modulated lightpattern at the detector track DETREF2 also comprises a modulated andspatially filtered imaged fringe pattern. It will be appreciated thatall of the spatially modulated light patterns move in tandem with thescale 310. In optical signal channels corresponding to each of thedetector tracks DETINC2, DETABS2, and DETREF2, individual photodetectorareas are arranged to spatially filter their respectively receivedspatially modulated light patterns to provide desirableposition-indicating signals (e.g., for the incremental scale trackpattern TINC2 producing quadrature signals, or other periodic signalshaving a spatial phase relationship that is conducive to signalinterpolation). In some embodiments, rather than individualphotodetector areas, a spatial filter mask with individual apertures maymask relatively larger photodetectors to provide light-receiving areasanalogous to the individual photodetector areas illustrated in order toprovide a similar overall signal effect according to known techniques.

In various applications, the detector electronics 320 and light source330 are mounted in a fixed relationship relative to one another, e.g.,in a readhead or gauge housing (not shown), and are guided along themeasuring axis relative to the scale 310 by a bearing system, accordingto known techniques. The scale may be attached to a moving stage, or agauge spindle, or the like, in various applications.

FIGS. 4A-4D illustrate various aspects related to the optical signalchannel corresponding to the incremental scale track pattern TINC2 ofFIG. 3. More specifically, FIG. 4A illustrates the illumination fringepattern IFP produced by the phase grating 350. The illumination fringepattern IFP is shown to have a pitch P_(MI) (e.g., 5 microns). FIG. 4Billustrates the incremental scale track pattern TINC2 which has a scalepitch P_(SF) (e.g., 4 microns). FIG. 4C is a graph of the resultingimage intensity signal IMG2 from the light from the combination of thefringe grating 350 and the incremental scale track TINC2 at the detectorplane DP. As shown in FIG. 4C, the resulting image intensity includesmoiré fringes with a beat frequency with an overall sinusoidal envelopepattern that has a modulated image pitch P_(IMESF) (e.g., 20 microns.)As described above, the image intensity has been spatially filtered(e.g., by the aperture 182) so as to filter out the high-frequencysignals HFS from the phase grating 350 and incremental scale trackpattern TINC2 in order to produce the approximately sinusoidal envelopesignal for the moiré imaged fringes, with the resulting modulated imagepitch P_(IMESF).

In various embodiments, the aperture 350 is configured such that theaperture width AW=F*λ*(a/(P_(MI)P_(SF)(P_(MI)−P_(SF)))), where a isgreater than 2.0 and less than 6.0. The spatially modulated image lightcomprises fringes (shown in detail in FIG. 4C) formed from theinterference of two diffractive orders which differ by a value Δn. Forexample, in some embodiments, if the moiré image intensity signal IMG2comes from the overlap of a +1 and a −1 diffractive order component ofthe scale light 332, then Δn=2. In other exemplary embodiments, Δn maybe 1 or 4. The image intensity signal IMG2 is modulated by an intensitymodulation envelope that has a spatial wavelength P_(IMESF) that dependson the scale pitch P_(SF) and the illumination fringe pitch P_(MI), andP_(SF) and P_(MI) are selected to cooperate with a detector pitch Pd ofthe detector track DETINC2 such thatΔnP_(MI)P_(SF)/(ΔnP_(MI)−P_(SF))=P_(IMESF)=m*Pd/k when the light sourceoutputs incoherent light andΔnP_(MI)P_(SF)/(2ΔnP_(MI)−P_(SF))=P_(IMESF)=m*Pd/k when the light sourceoutputs incoherent light, where m is a number of phase signals output bythe detector configuration and k is an odd integer, wherein the spatialwavelength P_(IMESF) is larger than the scale pitch P_(SF).

A series of vertical reference lines VRL drawn between FIGS. 4A, 4B, and4C provide an indication of signal levels from the illumination fringepattern of FIG. 4A that pass through the incremental scale track patternTINC2 of FIG. 4B, and appear as corresponding signal intensities in theresulting moiré image intensity of FIG. 4C. FIG. 4D is a diagram of theincremental detector track DETINC2 that, for purposes of illustration,has an image of the beat frequency envelope of the moiré image intensitysignal IMG2 from FIG. 4C superimposed upon it. As shown in FIG. 4D, thedetector track DETINC2 is connected so as to output quadrature signals,with four detector elements being within one period of the detectorpitch Pd, which also corresponds to one period P_(IMESF) of the moiréimage intensity signal IMG2.

FIGS. 5 and 6 show basic design reference information included in FIGS.26 and 27, respectively, of the previously incorporated U.S. Pat. No.7,186,969. The use of FIGS. 5 and 6 in relation to the selection ofaperture sizes in various embodiments may be understood based on thedisclosure of the '969 patent, and will not be described in detailherein. However, the related teachings may be used in the context ofthis disclosure. Much of the description of the '969 patent is in termsof incoherent illumination. One skilled in the art will make appropriateadaptations to its teachings based on known considerations related tothe differences between incoherent and coherent illumination in imagingsystems.

FIG. 7 is a partially schematic exploded diagram of an encoderconfiguration 700 that is one exemplary embodiment of a practicalimplementation of the encoder configuration 100 of FIG. 1. Certain ofthe components and operating principles of the encoder configuration 700are approximately similar to those of the encoder configuration 100 ofFIG. 1, and may generally be understood by analogy. For example, the 7XXseries numbers in FIG. 7 that have the same “XX” suffix as 1XX seriesnumbers in FIG. 1 may designate similar or identical elements, which mayfunction similarly, except as otherwise described or implied below.

As shown in FIG. 7, the encoder configuration 700 includes a scaleelement 710, an illumination system or portion 760, and a doublytelecentric imaging configuration 780. The illumination system orportion 760 includes a light source 730 (e.g., an LED) for emittingvisible or invisible wavelengths of light, a lens 740, and a beamsplitter 755. The doubly telecentric imaging configuration 780 comprisesa first lens array 781 at a first lens plane FLP, an aperture array 782in an aperture component 782′ at an aperture plane AP, a second lensarray 783 at a second lens plane SLP, and detector electronics 720 at adetector plane DP. The detector electronics 720 may be connected tosignal-generating and processing circuitry (not shown). The light source730 may also be connected to the signal-generating and processingcircuitry by power and signal connections (not shown).

With regard to the lens arrays 781 and 783 and the aperture array 782,it will be appreciated that each of these include individual elementsthat are similar to the first lens 181, aperture 182, and second lens183 of the encoder configuration 100 of FIG. 1. In FIG. 7, in each ofthe arrays, each of the individual elements similarly cooperate toprovide an individual image path or channel which may be referred to asan image channel or image channel configuration. Each of the imagechannels operates similarly to the image channel for the single lensesand aperture of the encoder configuration 100 described above withrespect to FIG. 1. In the embodiment of FIG. 7, the multiple imagechannels are utilized to provide additional levels of robustness for thesystem with respect to contamination, defects, scale waviness, etc., inthat if a single image channel becomes contaminated or otherwiseinhibited, the remaining image channels may still continue to provideaccurate imaging of the scale patterns.

In the embodiment shown in FIG. 7, the scale element 710 includes ascale pattern 715 that comprises the three scale track patternsdescribed above with respect to FIG. 1, including: the absolute scaletrack pattern TABS 1, the reference scale track pattern TREF1, and theincremental scale track pattern TINC1. In at least one embodiment, theabsolute scale track pattern TABS 1 may have a very “coarse” ABSresolution, on the order of the detector dimension along the X axis.

For the incremental scale track pattern TINC1, in at least oneembodiment, the incremental pitch may be relatively coarse (e.g., 20microns). As will be described in more detail below with respect to FIG.8, a finer pitch (e.g., 4 microns) may be implemented in a similarlysized encoder configuration in accordance with the principles disclosedherein. As will be described in more detail below with respect to FIG.10A, in at least one embodiment, the reference scale track pattern TREF1may comprise a series of reference marks, which may be formed as aseries of barker patterns, which may also serve as Vernier referencemarks, and which may be formed according to a variety of knowntechniques.

The detector electronics 720 includes a detector configuration 725comprising the three detector tracks DETABS1, DETREF1, and DETINC1,which are arranged to receive light from the three scale track patternsTABS1, TREF1, and TINC1, respectively. The detector electronics 720 mayalso include signal processing circuitry (e.g., signal offset and/orgain adjustments, signal-amplifying and combining circuits, etc.). In atleast one embodiment, the detector electronics 720 may be fabricated asa single CMOS IC.

In operation, light 731 (e.g., primary light) emitted from the lightsource 730 may be partially or fully collimated by the lens 740 and isdirected through the beamsplitter 755 over a beam area sufficient toilluminate the three scale track patterns TABS1, TREF1, and TINC1. Then,as exemplified by the image channel for the incremental scale trackpattern TINC1, the incremental scale track pattern TINC1 provides scalelight 732 that is redirected by the beam splitter 755 toward the lensarray 781. It will be appreciated that each limiting aperture of theaperture array 782, each of which has an aperture width AW along the Xdirection, acts as a spatial filter (as described above with respect toFIG. 2) to select or limit the light rays that pass through the givenimage channel for the incremental scale track pattern TINC1. As shown inFIG. 7, for each image channel, the corresponding lenses of the lensarray 781 transmit the light rays towards the corresponding apertures ofthe limiting aperture array 782. The corresponding apertures of thelimiting aperture array 782 then transmit the rays as spatially filteredimage light 733 to the respective lenses of the second lens array 783,and the respective lenses of the second lens array 783 transmit andfocus the spatially filtered image light to form respective spatiallymodulated light patterns corresponding to the respective portions of theincremental scale track pattern TINC1 at the respective portions of thedetector track DETINC1.

Thus, when the incremental scale track pattern TINC1 is illuminated, itoutputs a series of track-specific spatially modulated light patterns tothe respective portions of the detector track DETINC1 of the detectorelectronics 720, corresponding to each respective image channel. Animage of the spatially modulated light patterns is formed at an imageplane IMGP, which may be made to be coplanar with the detector trackDETINC 1.

Similar to the imaging of the spatially modulated light patterns fromthe incremental scale track pattern TINC1 on the detector track DETINC1,when the scale track patterns TREF1 and TABS1 are illuminated by thelight from the lens 740, they output track-specific spatially modulatedlight patterns (e.g., patterned light corresponding to their patterns)to the track-specific detector tracks DETREF1 and DETABS1, respectively,of the detector electronics 720. As noted above, the reference scaletrack pattern TREF1 (e.g., with barker patterns) can be resolved toindicate a particular incremental wavelength, so that the wavelengthfrom the incremental scale track pattern TINC1 is not ambiguous relativeto the absolute mark from the absolute scale track pattern TABS1. Itwill be appreciated that all of the spatially modulated light patternsmove in tandem with the scale 710. In each of the detector tracksDETINC1, DETABS1. and DETREF1, individual photodetector areas arearranged to spatially filter their respectively received spatiallymodulated light patterns to provide desirable position-indicatingsignals (e.g., the incremental detector track DETINC1 producingquadrature signals, or other periodic signals having a spatial phaserelationship that is conducive to signal interpolation).

In various applications, the detector electronics and light source aremounted in a fixed relationship relative to one another, e.g., in areadhead or gauge housing (not shown), and are guided along themeasuring axis relative to the scale 710 by a bearing system, accordingto known techniques. The scale may be attached to a moving stage, or agauge spindle, or the like, in various applications. The configurationshown in FIG. 7 is a reflective configuration. That is, the light sourceand the detector electronics are arranged on the same side of the scale710, and positioned for angled illumination and reflection, according toknown techniques. Thus, the scale pattern 715 comprises light-absorbingportions and light-reflecting portions (e.g., fabricated on a substrateusing known reflecting techniques) that output the spatially modulatedlight patterns to the detector tracks by reflection. It will beappreciated that similar components may be arranged in transmissiveembodiments (e.g., see FIG. 1).

FIG. 8 is a partially schematic exploded diagram of an encoderconfiguration 800 that is one exemplary embodiment of a practicalimplementation of the encoder configuration 300 of FIG. 3. Certain ofthe components and operating principles of the encoder configuration 800are approximately similar to those of the encoder configuration 300 ofFIG. 3, and may generally be understood by analogy. For example, the 8XXseries numbers in FIG. 8 that have the same “XX” suffix as 3XX seriesnumbers in FIG. 3 may designate similar or identical elements, which mayfunction similarly, except as otherwise described or implied below.

As shown in FIG. 8, the encoder configuration 800 includes a scaleelement 810, an illumination system or portion 860, and a doublytelecentric imaging configuration 880. The illumination system orportion 860 includes a light source 830 (e.g., an LED) for emittingvisible or invisible wavelengths of light, a lens 840, a phase grating850, and a beam splitter 855. As will be described in more detail below,the phase grating 850 may, in at least one embodiment, be placed andsized within the image channels for the incremental and reference scaletrack patterns TINC2 and TREF2, but not the absolute scale track patternTABS2. The doubly telecentric imaging configuration 880 comprises afirst lens array 881 at a first lens plane FLP, an aperture array 882 atan aperture plane AP, a second lens array 883 at a second lens planeSLP, and a detector electronics 820 at a detector plane DP. It will beappreciated that the lens arrays 881 and 883, and the aperture array882, are arranged and operate similarly to the lens arrays 781 and 783and the aperture array 782 described above with respect to FIG. 7. Thedetector electronics 820 may be connected to signal-generating andprocessing circuitry (not shown). The light source 830 may also beconnected to the signal-generating and processing circuitry by power andsignal connections (not shown).

In the embodiment shown in FIG. 8, the scale element 810 includes ascale pattern 815 that comprises the three scale track patternsdescribed above with respect to FIG. 3, including the absolute scaletrack pattern TABS2, the reference scale track pattern TREF2, and theincremental scale track pattern TINC2. In at least one embodiment, theabsolute scale track pattern TABS2 may have a relatively “coarse” ABSresolution, on the order of the detector dimension along the X-axis. Asdescribed above with respect to FIG. 3, the reference scale trackpattern TREF2 and the incremental scale track pattern TINC2 are utilizedand imaged in accordance with the spatial filtering and imagingprinciples disclosed herein.

As shown in FIG. 8, the detector electronics 820 includes a detectorconfiguration 825 comprising the three detector tracks DETABS2, DETREF2,and DETINC2, arranged to receive light from the three scale trackpatterns TABS2, TREF2, and TINC2, respectively. The detector electronics820 may also include signal processing circuitry (e.g., signal offsetand/or gain adjustments, signal-amplifying and combining circuits,etc.). In at least one embodiment, the detector electronics 820 may befabricated as a single CMOS IC.

In operation, light 831 (e.g., primary light) emitted from the lightsource 830 may be partially or fully collimated by the lens 840 anddirected through the beam splitter 855 over a beam area sufficient toilluminate the three scale track patterns TABS2, TREF2, and TINC2. Thephase grating 850 is sized to diffract the source light to providediffracted structured light 831′ to the reference and incremental scaletrack patterns TREF2 and TINC2 (but not the absolute scale track patternTABS2). Then, as exemplified by the image channel for the incrementalscale track pattern TINC2, the incremental scale track pattern TINC2outputs scale light 832 that is redirected by the beam splitter 855toward the lens array 881. It will be appreciated that each limitingaperture of the aperture array 882, each of which has an aperture widthAW along the X direction, acts as a spatial filter (as described abovewith respect to FIG. 4) to select or limit the light rays that passthrough the given image channels. In other words, as described above,the spatial filtering effectively blurs out the high-frequency portionsof the images produced by the phase grating and incremental scale trackpatterns, so that the remaining signal primarily consists of themodulation which may be thought of as the beat frequency between thefringe pitch of the structured illumination and the pitch of the scalegrating. The resulting modulated image pitch is a measurement of theperiod of that beat frequency envelope.

As shown in FIG. 8, for each image channel, the corresponding lenses ofthe lens array 881 transmit the light rays towards the correspondingapertures of the limiting aperture array 882. The correspondingapertures of the limiting aperture array 882 transmit the rays asspatially filtered image light 833 to the respective lenses of thesecond lens array 883, and the respective lenses of the second lensarray 883 transmit and focus the spatially filtered image light to formrespective spatially modulated light patterns corresponding to therespective portions of the incremental scale track pattern TINC2 at therespective portions of the detector track DETINC2. As described abovewith respect to FIG. 4, in accordance with the principles disclosedherein, the spatially modulated light patterns at the detector trackDETINC2 comprise modulated and spatially filtered imaged fringepatterns.

Similarly, when the scale track patterns TREF2 and TABS2 areilluminated, they output track-specific spatially modulated lightpatterns to the track-specific detector tracks DETREF2 and DETABS2,respectively, of the detector electronics 820. As noted above, thespatially modulated light patterns at the reference detector trackDETREF2 also comprise modulated and spatially filtered imaged fringepatterns. It will be appreciated that all of the spatially modulatedlight patterns move in tandem with the scale 810. In optical signalchannels corresponding to each of the detector tracks DETINC2, DETABS2,and DETREF2, individual photodetector areas are arranged to spatiallyfilter their respectively received spatially modulated light patterns toprovide desirable position-indicating signals (e.g., the incrementaldetector track DETINC2 producing quadrature signals, or other periodicsignals having a spatial phase relationship that is conducive to signalinterpolation).

In various applications, the detector electronics 820 and light source830 are mounted in a fixed relationship relative to one another, e.g.,in a readhead or gauge housing (not shown), and are guided along themeasuring axis relative to the scale 810 by a bearing system, accordingto known techniques. The scale may be attached to a moving stage, or agauge spindle, or the like, in various applications. The configurationshown in FIG. 8 is a reflective configuration. That is, the light source830 and the detector electronics 820 are arranged on the same side ofthe scale 810 and positioned for angled illumination and reflection,according to known techniques. Thus, the scale pattern 815 compriseslight-absorbing portions and light-reflecting portions (e.g., fabricatedon a substrate using known techniques) that output the spatiallymodulated light patterns to the detector tracks by reflection. It willbe appreciated that similar components may be arranged in transmissiveembodiments (e.g., see FIG. 3).

FIG. 9 is a diagram of an encoder configuration 900 illustrating analternative embodiment of the phase grating portion of the encoderconfiguration 800 of FIG. 8. As shown in FIG. 9, the encoderconfiguration 900 includes a scale element 910, a light source 930, alens 940, two phase gratings 950A and 950B, and a beam splitter 955. Aprimary difference from the encoder configuration 800 of FIG. 8 is that,rather than utilizing a single phase grating 850, the encoderconfiguration 900 utilizes two phase gratings 950A and 950B. In onespecific example embodiment, the phase grating 950A may be a 0.92 micronphase grating, while the phase grating 950B may be a 0.84 micron phasegrating with air gap (no coupling). This configuration allows for acompact design, in that the phase grating 950B does not require thelight beams output by the phase grating 950A to completely separate. Inone specific example embodiment, after the transmission of the lightthrough the phase gratings 950A and 950B, light fringes are producedwith a specified period (e.g., 5 microns), which in combination with thepitch of the incremental scale track pattern TINC2 (e.g., 4 microns),produces modulated and spatially filtered fringes with a specifiedperiod (e.g., 20 microns.)

FIGS. 10A and 10B are diagrams of the scale track pattern arrangementsof the encoder configurations of FIGS. 1 and 3, respectively. As shownin FIG. 10A, the scale track pattern arrangement 1000A includes theabsolute scale track pattern TABS1, the reference scale track patternTREF1, and the incremental scale track pattern TINC1. As describedabove, the absolute scale track pattern TABS1 provides signals usable todetermine an absolute position over an absolute measuring range, which,in the embodiment of FIG. 10A, are illustrated as including coded signalportions which indicate absolute positions along the scale trackpattern.

For the incremental scale track pattern TINC1, the incremental pitch isillustrated as being relatively coarse (e.g., 20 microns.) In thesection of the reference scale track pattern TREF1 shown in FIG. 10A,four reference mark patterns RM1A-RM1D are illustrated, and are shown tooccur at specified intervals. In at least one embodiment, the referencemarks may be formed as barker patterns, which may be formed according toa variety of known techniques. The reference mark patterns may alsofunction as Vernier reference marks. As described above, the referencescale track pattern TREF1 is able to be resolved to a level that allowsit to indicate a particular incremental wavelength, so that theincremental wavelength (e.g., from the incremental scale track patternTINC1) is not ambiguous relative to an absolute mark (e.g., from theabsolute scale track pattern TABS1). As shown in FIG. 10A, the scale hasan overall width dimension X1, while the area covered by the scale trackpatterns TABS1, TREF1, and TINC1 has a width dimension X2. In onespecific example embodiment, the dimension X1 is equal to 13millimeters, while the dimension X2 is equal to 3.9 millimeters.

As shown in FIG. 10B, the scale track pattern arrangement 1000B includesthe absolute scale track pattern TABS2, the reference scale trackpattern TREF2, and the incremental scale track pattern TINC2. Thevarious possible dimensions and configurations for the scale trackpatterns will be described in more detail below with respect to FIG. 11.In general, it will be appreciated that the scale track patternarrangement 1000B is designed to be approximately the same size as thescale track pattern arrangement 1000A of FIG. 10A, such that the scaletrack pattern arrangement 1000B can be substituted into an encoderconfiguration that is otherwise designed for the scale track patternarrangement 1000A. As shown in FIG. 10B, the absolute scale trackpattern TABS2 provides signals usable to determine an absolute positionover an absolute measuring range, and may comprise coded portionssimilar to those of the absolute scale track pattern TABS1 of FIG. 10A.In at least one embodiment, the absolute scale track pattern TABS2 mayhave a very coarse ABS resolution, on the order of the detectordimension along the X axis.

As shown in FIG. 10B, the incremental scale track pattern TINC2 isillustrated as having a much finer pitch (e.g., 4 microns) as comparedto the pitch of the incremental scale track pattern TINC1 of FIG. 10A(e.g., 20 microns). The portion of the reference scale track patternTREF2 shown in FIG. 10B is illustrated as including a series of fourreference mark patterns RM2A-RM2D. The reference mark patterns RM2A-RM2Dmay be formed as barker patterns, according to a variety of knowntechniques. The reference mark patterns may also function as Vernierreference marks. The reference scale track pattern TREF2 is designed soit can be resolved to a level that allows it to indicate a particularincremental wavelength for the incremental scale track pattern TINC2, sothat the incremental wavelengths are not ambiguous relative to anabsolute mark from the absolute scale track pattern TABS2. In at leastone embodiment, the combination of the modulated and spatially filteredimages of the reference track pattern TREF2 and the incremental trackpattern TINC2 create a synthetic wavelength for which the measuredsynthetic phase points to the correct incremental scale track patterncycle (e.g., a measured synthetic phase of zero may indicate a correctincremental cycle corresponding to that phase).

As an example, in the embodiment of FIG. 10B, each of the reference markpatterns RM2A-RM2D is shown to have a corresponding phase markerPHS2A-PHS2D, which indicates a point at which a perfectly aligned phasewould occur for each position. In other words, in the reference scaletrack pattern TREF2, the reference mark patterns (e.g., patternsRM2A-RM2D) are provided at selected intervals (e.g., 0.6 millimeters).The phase of each reference mark pattern (e.g., at the center of eachpattern where the phase markers PHS2A-PHS2D occur) matches (or has aconstant phase offset from) the phase of the incremental scale trackpattern TINC2 at the locations that are the specified distance (e.g.,0.6 millimeters) apart. The synthetic phase of the incremental scaletrack pattern TINC2 and the reference scale track pattern TREF2 islarger than the reference mark pattern length (i.e., is larger than thelength of each of the individual barker patterns).

As described above, the reference scale track pattern TREF2 (with thereference mark patterns) is designed to produce the same kind ofmodulated and spatially filtered images as the incremental scale trackpattern TINC2. In order to produce the modulated and spatially filteredimaging, a phase grating is utilized that has a pitch that is close tothe pitch of the incremental scale track pattern TINC2 and the referencescale track pattern TREF2 (e.g., a phase grating pitch of 5 microns ascompared to an incremental scale track pitch of 4 microns, and areference scale track pitch of 4.1 microns). The resulting modulated andspatially filtered imaged fringe period from the phase grating and theincremental scale track pattern TINC2 may be relatively coarse (e.g., 20microns) and may be slightly different than the modulated and spatiallyfiltered imaged fringe period produced by the phase grating andreference scale track pattern TREF2 (e.g., 22.77 microns).

By making the reference scale track pattern TREF2 have a slightlydifferent pitch (e.g., 4.1 microns) as compared to the pitch of theincremental scale track pattern TINC2, (e.g., 4.0 microns) the referencescale track pattern phase will match the incremental scale track patternphase only at one specified point along a specified length (e.g., onlymatches at one point along a barker pattern length within the referencescale track pattern TREF2, as indicated by the phase markersPHS2A-PHS2D). This position where the phases match defines a particularincremental wavelength for the incremental scale track pattern TINC2.

As described above, by utilizing an incremental scale track pattern witha relatively fine pitch (e.g., 4 microns) which is imaged by structuredlight that is produced by a phase grating with a selected pitch (e.g., 5microns), a modulated and spatially filtered pattern with a relativelycoarse modulated image pitch (e.g., 20 microns) can be produced. It willbe appreciated that in such an embodiment, a selected ratio (e.g., 5to 1) exists between the modulated image pitch (e.g., 20 microns) andthe pitch of the incremental scale track pattern (e.g., 4 microns). Inselected embodiments, ratios of approximately 5 to 1 or higher (e.g., 10to 1, 20 to 1, etc.) may be desired in order to allow a high resolutionincremental scale track pattern to be utilized in an encoderconfiguration that was previously designed for a coarser incrementalscale track pitch.

FIG. 11 is a table 1100 illustrating the parameters for various scaleand detector track combinations for the encoder configuration of FIG. 3.As shown in FIG. 11, for a first implementation, the incremental scaletrack pattern TINC2 is indicated as having a pitch of p=4 microns, andthe associated phase grating creates structured light of a fringe periodS=5 microns. The imaged fringe period resulting from the modulated andspatially filtered imaging is f=20 microns. An interpolation factor(which indicates the needed level of interpolation) is K=40. Thedetector elements are designated as having a pitch d=15 microns. It willbe appreciated that in certain embodiments, the detector element pitchmay be designated as being ¼, ⅓, ⅔, or ¾ of the fringe period f. In atleast one embodiment, the detector element pitch may be made to be ¾ fora 20 micron fringe (as for the detector element pitch d=15 microns inthe present example).

For the reference scale track pattern TREF2 in the first implementation,the pitch of the elements within each of the barker patterns is p′=4.1microns, while the associated phase grating creates structured lightwith a fringe period S=5 microns (similar to that for the incrementalscale track pattern). The imaged fringe period produced by thecombination of the structured light from the phase grating through thereference scale track pattern produces a modulated and spatiallyfiltered imaged fringe period f′=22.77 microns. The interpolation factoris K=40. The pitch of the detector elements is d′=17 microns. For thecombined use of the incremental and reference scale track patterns, theVernier synthetic wavelength (ff′/(f−f′)) equals 164 microns. The lengthof each of the barker patterns within the reference scale track patternis L=136 microns (with 33 lines with the pitch p′=4.1 microns). It willbe appreciated that in certain embodiments, the number of lines in thebarker pattern may be required in order to form an adequately visiblefringe (i.e., a significant enough portion of the beat frequencyenvelope), so that it can be properly detected as part of the modulatedand spatially filtered image produced at the detector tracks. Withregard to the number of detector elements in the image array per trackand region, and their total length, for the incremental detector trackDETINC 1 there are 8 elements in each set (with a 120 micron totallength), and for the reference detector track DETREF1, there are 8elements in each set (with a 136 micron total length). The number ofincremental cycles between the barker patterns is 150.

As shown in FIG. 11, for a second implementation, the incremental scaletrack pattern TINC2 is indicated as having a pitch of p=8 microns, andthe associated phase grating creates structured light with a fringeperiod S=10 microns. The imaged fringe period resulting from themodulated and spatially filtered imaging is f=40 microns. Theinterpolation factor is K=27.6. The detector elements are designated ashaving a pitch d=10 microns. In at least one embodiment, the detectorelement pitch may be made to be ¼ for a 40 micron fringe (as for thedetector element pitch d=10 microns in the present example).

For the reference scale track pattern TREF2 in the secondimplementation, the pitch of the elements within each of the barkerpatterns is p′=8.3 microns, while the associated phase grating createsstructured light with a fringe period S=10 microns (similar to that forthe incremental scale track pattern). The imaged fringe period producedby the combination of the structured light from the phase gratingthrough the reference scale track pattern produces a modulated andspatially filtered imaged fringe period f′=48.8 microns. Theinterpolation factor is K=27.6. The pitch of the detector elements isd′=12.2 microns. For the combined use of the incremental and referencescale track patterns, the Vernier synthetic wavelength (ff′/(f−f′))equals 221.3 microns. The length of each of the barker patterns withinthe reference scale track pattern is L=approximately 195 microns (withapproximately 23 lines with the pitch p′=4.1 microns.) With regard tothe number of detector elements in the image array per track and regionand their total length, for the incremental detector track DETINC2,there are 16 elements in each set (with a 160 micron total length), andfor the reference detector track DETREF2, there are 16 elements in eachset (with a 195 micron total length). The number of incremental cyclesbetween the barker patterns is 75.

FIG. 12 is a substantial copy of a figure included in U.S. patentapplication Ser. No. 12/535,561 (the '561 application) which ispublished as U.S. Patent Application Publication No. US 2011/0031383 A1(the '383 publication), and which is hereby incorporated herein byreference in its entirety. FIG. 12 may be understood based on thedisclosure of the '561 application, and will not be described in detailherein. However, the related teachings may be used in the context of theprinciples disclosed herein.

Briefly, FIG. 12 is a schematic cross-sectional diagram 1200 showingdiffering optical paths through an image channel 1280-1 of a doublytelecentric encoder imaging arrangement 1270-1 which is analogous to thedoubly telecentric imaging configurations 380, 880, and 1380 shownherein. U.S. Pat. No. 7,307,789 (the '789 patent), which is herebyincorporated herein by reference, discloses various embodiments ofdoubly telecentric encoder configurations which utilize a second lens(or lens array) that is similar in form to a first lens (or lens array),and which is inverted relative to the first lens along an optical axis,such that lens aberrations of the two similar lenses approximatelycompensate one another to reduce aberrations in the resulting image. Itshould be appreciated that the teachings of the '789 patent address onlycompensating lens aberrations that cause spatial distortions in an imageof a scale pattern; that is, distortion of the location of patternfeatures in the image. The embodiment shown in FIG. 12 may provide asimilar type of correction of spatial distortions in an image when thefirst lens 1210-1 and second lens 1210-1′ have similar aberrations.However, a more subtle problem may occur, related to interferenceeffects that may appear in the image due to lens aberrations. The '789patent does not address this problem. The '561 application does addressthis problem, and its teachings are applicable in various embodimentsherein, especially those teachings related to diffracted order rayblocking and aperture dimensions, which may be applied with appropriateadaptations in some embodiments according to the principles disclosedherein.

FIGS. 13A and 13B show a configuration 1300 that is another exemplaryembodiment of a practical implementation of the encoder configurationaccording to the principles disclosed herein. Certain of the componentsand operating principles of the encoder configuration 1300 areapproximately similar to those of the encoder configuration 300 of FIG.3 and/or 800 of FIG. 8, and may generally be understood by analogy. Forexample, the 13XX series numbers in FIG. 13 that have the same “XX”suffix as 3XX series numbers in FIG. 3 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below or in FIGS. 13A and 13B. In at least oneembodiment, the dimensional relationships of the layout shown in FIGS.13A and 13B are shown in realistic exemplary proportions relative to oneanother, although such relationships may be changed in various otherembodiments. In at least one embodiment, for reference, the dimensionDIMZ may be approximately 26.5 mm, and the dimension DIMY may beapproximately 48 mm. The dimension GAP may be approximately 1 mm. Otherapproximate dimensions may be scaled based on these dimensions, in oneexemplary embodiment. It will be appreciated that this embodiment isexemplary only, and not limiting.

As shown in FIG. 13A, the encoder configuration 1300 includes a scaleelement 1310, an illumination system or portion 1360, and a doublytelecentric imaging configuration 1380. The illumination system orportion 1360 includes a light source 1330 (e.g., a laser diode, LED, orthe like) for emitting visible or invisible wavelengths of light 1331(e.g., 655 micron wavelength, for a laser, in at least one embodiment),an aperture 1335, a collimating lens 1340 (or approximately collimating,at least in the XY plane), a polarizing beam splitter 1390, a beam dump1392, a reflector 1342, an aperture element 1345, a reflector 1344, aphase grating 1350, and a beam splitter 1355. The doubly telecentricimaging configuration 1380 comprises a first lens 1381 at a first lensplane FLP, an aperture 1382 in an aperture component 1382′ at anaperture plane AP, a second lens 1383 at a second lens plane SLP anddetector electronics 1320 at a detector plane DP. The detectorelectronics 1320 may be connected to signal-generating and processingcircuitry (not shown). The light source 1330 may be connected to thesignal-generating and processing circuitry by power and signalconnections (not shown).

In operation, light 1331 (e.g., primary light) emitted from the lightsource 1330 is transmitted through the aperture 1335, which may blockstray portions of the light 1331. In at least one embodiment, theaperture 1335 may have a diameter of 4 mm. The transmitted light may benearly or fully collimated by the lens 1340 and is directed through thebeam splitter 1341. Z-polarized light is passed by the polarizingbeamsplitter 1390 as light 1331Z. The polarizing beam splitter 1390 isconfigured to prevent stray light from being reflected back to the lightsource 1330. Such stray light is reflected by the polarizing beamsplitter 1390 as a beam 1391 which is directed to a beam dump 1392.

The light 1331Z passes through a quarter wave plate 1393, which convertsZ-polarized incident light to R circularly polarized light 1331C. Lightwhich may be reflected along the optical path by subsequentlyencountered elements returns as L circularly polarized light, andbecomes X-polarized as it passes back through the quarter wave plate1393. Such X-polarized reflected light is blocked by the polarizingbeamsplitter 1390, and directed to the beam dump 1392, such that it doesnot return to disrupt the light source 1330 or create other extraneouslight rays.

The light 1331C is reflected by the reflector 1342 and directed throughthe aperture element 1345 which shapes the light beam 1331C such that itwill illuminate a desired portion (e.g., a desired track portion) of thescale 1310 after it is reflected by the reflector 1344 and passesthrough the phase grating 1350 to become the diffracted structured light1331′. In at least one embodiment, the aperture 1345 may have an Xdimension of 6 mm and a Y dimension of 1.5 mm.

In at least one embodiment where the light source 1330 is a laser diodeemitting 655 micron wavelength light, the scale element may have agrating pitch of 4.00 microns and the phase grating 1350 may have agrating pitch of 4.44 microns and be configured to block zero orderlight. The resulting amplitude modulation may have a period ofapproximately 20 microns.

Then, the scale element 1310 reflects the diffracted structured lightfrom its scale grating elements to provide the scale light 1332, whichincludes the previously described modulation, and is directed throughthe beam splitter 1355 to be imaged onto the detector 1320 by the doublytelecentric imaging configuration 1380, which may function according topreviously outlined principles, to spatially filter the scale light1332, such that the period of the amplitude modulation, whichapproximately matches the spatial filtering period of the detectorelements of the detector 1320, is the primary intensity modulation ofthe scale light 1332 that finally causes the signal variation in thesignals of the detector 1320. In at least one embodiment, the aperture1382 of the doubly telecentric imaging configuration 1380 may have adiameter of approximately 1 mm in order to block zero order and +first-order components of the scale light 1332 and provide the desiredfiltering of the spatial frequency components of the scale light 1332that have a higher spatial frequency than the amplitude modulationcomponent. Another way of describing this is that the aperture 1382 isconfigured to prevent imaging of the phase grating and/or the scalegrating.

FIG. 14 shows a reference diagram of various beam paths in an exemplaryembodiment of an encoder configuration 1400 which includes a coherentlight source. Certain of the components and operating principles of theencoder configuration 1400 are approximately similar to those of theencoder configuration 300 of FIG. 3 and/or 800 of FIG. 8, and maygenerally be understood by analogy. For example, the 14XX series numbersin FIG. 14 that have the same “XX” suffix as 3XX series numbers in FIG.3 may designate similar or identical elements, which may functionsimilarly, except as otherwise described or implied below or in FIG. 14.As shown in FIG. 14, the light source emits source light 1431. A phasegrating 1450 splits the source light into structured illumination 1431′comprising various diffractive order ray bundles. FIG. 14 shows +1 orderray bundle 1431 p and −1 order ray bundle 1431 p that interfere toprovide an illumination fringe pitch Pi. It should be appreciated thatadditional orders of ray bundles are present in the structuredillumination 1431′. However, only the +1 order and −1 order are shown inFIG. 14 for the sake of simplicity. The scale 1410 comprises a scalepitch Pg. The scale 1410 receives the structured illumination 1431′ andoutputs scale light 1432 comprising fringes with an envelope comprisinga period Pe. The period Pe may be derived in terms of the scale fringepitch Pi and the scale pitch Pg as Pe=PgPi/(2Pi−Pg). It should beappreciated that the denominator contains a term 2Pi, which is Pi in thecase of incoherent light.

FIG. 15 is a partially schematic exploded diagram of an encoderconfiguration 1500 that includes an alternative embodiment of anillumination portion 1560. With the exception of the illuminationportion 1560, the components and operating principles of the encoderconfiguration 300 are approximately similar to those of the encoderconfiguration 300 of FIG. 3, and may generally be understood by analogy.For example, 15XX series numbers in FIG. 15 that have the same “XX”suffix as 3XX series numbers in FIG. 3 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below. In the embodiment shown in FIG. 15, theencoder configuration 1500 is configured such that the illuminationportion 1560 (more specifically, the aperture configuration 1572)suppresses or eliminates the transmission of unwanted orders of light tothe scale grating 1510 and only allows the wanted orders (e.g., only ±1orders). This improves signal quality of the encoder configuration 1500,in comparison to previously disclosed configurations. The inventors havedetermined that for the encoder configurations disclosed herein,residual orders of light lead to periodic irregularities in theresulting signals at the detectors, including irregularities related tozero order light in the structured illumination interacting with thescale grating. It has been found that such irregularities may appear inalternate periods of the signals at the detectors, rather than appearingin every period of the signal, which reduces the ability to accuratelycompensate and/or interpolate the resulting signals. The illuminationconfigurations disclosed below are therefore of particular value incombination with the encoder configurations taught herein, althoughtheir utility is not limited to such configurations.

In addition to the components 1530, 1540, and 1550, the operation ofwhich may be understood based on the previous description of analogouscomponents, the illumination portion 1560 further comprises a firstfiltering lens 1571, a spatial filtering aperture configuration 1572positioned approximately at a focal plane of the first filtering lens1571, a second filtering lens 1573 positioned at a plane which islocated approximately at a distance equal to its focal length from thespatial filtering aperture configuration 1572. In operation, theillumination grating 1550 outputs diffracted structured light 1531′ in amanner similar to FIG. 3. The diffracted structured light 1531′ isfocused by the first illumination lens 1571 at a plane of the spatialfiltering aperture configuration 1572. The spatial filtering apertureconfiguration 1572 is configured to block zero order diffracted lightfrom the diffracted structured light 1531′ using a central portion 1572c and to block higher order diffracted light with edges of the apertureconfiguration 1572, and transmit spatially filtered structuredillumination 1531″ which comprises only +1 and −1 order diffracted lightcomponents using an open aperture portion 1572 op. In the embodimentshown in FIG. 15, the spatial filtering aperture configuration 1572 isconfigured with the central portion 1572 c surrounded by the openaperture portion 1572 op comprising symmetrically placed slits. In somealternative embodiments, a spatial filtering aperture configuration maycomprise a central circular stop surrounded by an annular aperture. Thesecond illumination lens 1573 receives the spatially filtered structuredlight 1531 and outputs the spatially filtered structured illumination1531″ which consists of the +1 and −1 order diffracted light componentsto the scale pattern 1515 at a plane coinciding with the scale grating1510. The dimensions along the measuring axis direction of the centralportion 1572 c and the open aperture portion 1572 op may be determinedby analysis or experiment, and are generally selected to block zeroorder light and transmit +/− first-order diffracted light in thisparticular example, respectively, as outlined above.

It should be appreciated that zero order light output from the scalegrating 1510 may be suppressed or eliminated by the imaging portion1580, or more specifically the aperture configuration limiting aperture1582 may be configured to spatially filter zero order light. However, inexemplary embodiments, the first filtering lens 1571 may focus light tothe spatial filtering aperture configuration 1572 with an angle (e.g.,1°) about ten times larger than the angle at the limiting aperture 1582(e.g., 0.1°). In such embodiments, the limiting aperture 1582 is muchmore sensitive to misalignment than the spatial filtering apertureconfiguration 1572. Therefore, it is more advantageous to block zeroorder light within the illumination portion 1560 than in the imagingportion 1580. Furthermore, in the embodiment shown in FIG. 15, theillumination grating 1550 may be an amplitude grating (in contrast withthe phase grating 350), which reduces the cost of manufacturing theencoder configuration 1500. In the embodiment shown in FIG. 3, theencoder configuration 300 has a practical limit on how far the scalepattern 315 may be placed from the phase grating 350. If the scalepattern 315 is placed too far away from the phase grating 350, then twodiffractive orders (e.g., +1 and −1 orders) may not overlap and will notinterfere in order to provide diffracted structured light 331′. However,the illumination portion 1560 is configured to image diffractedstructured light 1531′ from a plane approximately coinciding with theillumination grating 1550. This allows for a larger operating gap thanthe encoder configuration 300. This also allows for the illuminationportion 1560 to more efficiently provide source light because theinterfering orders overlap the most at the plane of the illuminationgrating 1550. If desired, the modulated image pitch P_(IMESF) may beadjusted by selecting an appropriate combination of the first filteringlens 1571 and the second filtering lens 1573 with focal lengths selectedto give a desired pitch P_(MI) of the illuminated fringe pitch patternIFP. Compared to the configuration disclosed in U.S. patent applicationSer. No. 13/717,586, this provides an extra degree of design freedom forconfiguring the encoder 1500 such that the resulting modulated imagepitch of the spatially filtered pattern matches the pitch of apredetermined given detector. The modulated image pitch P_(MI) may bechosen, for example, to provide desired relationships between the scalepitch P_(SF) and the modulated image pitch P_(IMESF) according toprinciples outlined in FIGS. 4A-4D.

FIG. 16 is a drawing of an illumination portion 1660 which may be usedin a reflective encoder configuration 1600 according to the principlesdisclosed herein. The components and operating principles of the encoderconfiguration 1600 are approximately similar to those of the encoderconfiguration 1500 of FIG. 15, and may generally be understood byanalogy. For example, 16XX series numbers in FIG. 16 that have the same“XX” suffix as 15XX series numbers in FIG. 15 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below. In the embodiment shown in FIG. 16, theillumination portion 1660 additionally comprises a polarizer 1676, apolarizing beam splitter 1677, and a quarter wave plate 1678. Incontrast with the encoder configuration 1500 which is a transmissiveconfiguration, the encoder configuration 1600 is a reflectiveconfiguration. The transmissive configuration 1500 may be readilyadapted to a reflective configuration analogous to that described withreference to FIG. 16, or by analogy with other reflective configurationsdisclosed herein. However, the addition of polarizers according to theprinciples taught with reference to FIG. 16 provides a more efficientuse of light, as described in greater detail below. Such a use ofpolarizers may be adapted for use in conjunction with any compatibleembodiment described herein.

In operation, the polarizer 1676 inputs coherent collimated light 1631from a lens 1640 and outputs collimated light which is linearlypolarized. The polarizing beamsplitter 1677 reflects spatially filteredstructured illumination 1631″ from the second filtering lens 1673 to thequarter wave plate 1678. The quarter wave plate 1678 circularlypolarizes the spatially filtered structured illumination 1631″ from thesecond filtering lens 1673 and outputs it to the scale pattern 1615. Thescale pattern 1615 reflects spatially modulated image light 1632 with acircular polarization to the quarter wave plate 1678. The quarter waveplate 1678 inputs the spatially modulated image light 1632 with acircular polarization and outputs the spatially modulated image light1632 to the polarizing beam splitter 1677 with a linear polarizationwhich is rotated 90 degrees with respect to the structured illumination1631″ input from the second filtering lens 1673. The polarizing beamsplitter 1677 transmits the spatially modulated image light 1632 with alinear polarization to the imaging portion 1680. Because the quarterwave plate 1678 returns spatially modulated image light 1632 to thepolarizing beams splitter 1677 with a polarization rotated by 90 degrees(thus matching the polarization direction to transmit through the beamsplitter 1677), this increases the amount of light output to the imagingportion 1680 by a factor of four compared to a configuration lacking thequarter wave plate 1678.

FIG. 17 is a drawing of an illumination portion 1760 which may be usedin an encoder configuration 1700 according to the principles disclosedherein. The components and operating principles of the encoderconfiguration 1700 are approximately similar to those of the encoderconfiguration 300 of FIG. 3, and may generally be understood by analogy.For example, 17XX series numbers in FIG. 17 that have the same “XX”suffix as 3XX series numbers in FIG. 3 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below.

In the embodiment shown in FIG. 17, the encoder configuration 1700 isconfigured such that the illumination portion 1760 transmits only wantedorders (e.g., only ±1 orders) to a portion of a scale track 1715 whichis imaged by an imaging configuration 1780 to a detector configuration1725. As shown in FIG. 17, a light source 1730 is configured to outputsource light 1731 having a wavelength λ. In some embodiments, the lightsource 1730 may comprise one of a laser diode, a spatially coherent LEDand a series of independent linear sources arranged perpendicular to themeasuring axis direction 82 (e.g., an LED point source masked with slitsor a grating with spacing or a period that provides spatially coherentsource light 1731 linear sources that contribute cumulatively to aninterference fringe pattern IFP). A collimation portion 1740 (i.e., acollimating lens) is arranged to collimate the source light 1731. Astructured illumination generating portion 1770 is configured to inputthe source light 1731 and provide structured illumination 1731′, whereinthe structured illumination 1731′ comprises an illumination fringepattern IFP oriented transversely to the measuring axis direction 82which is input to the scale track 1715. The scale track 1715 isconfigured to spatially modulate the input illumination fringe patternIFP and output scale light comprising spatially modulated image light.The detector configuration 1725 and imaging configuration 1780 areconfigured such that only scale light arising from an imaged region IRof the scale track 1715 is imaged to the detector configuration 1780.The operations of the imaging configuration 1780 and the detectorconfiguration 1725 are analogous to those of the imaging configuration380 and the detector configuration 325 and may be understood by analogyto the descriptions thereof. The structured illumination generatingportion 1770 comprises a beam separating portion 1771 and anillumination grating 1750. The beam separating portion comprises a beamsplitter 1777 and a reflector 1778. In the particular embodiment shownin FIG. 17, the collimation portion 1740 is located between the lightsource 1730 and the beam separating portion 1771, but in otherembodiments a collimation portion may be located in other positionsbetween a light source and an illumination grating. For example, inalternative embodiments, the collimation portion 1740 may be locatedbetween the beam separating portion 1771 and the illumination grating1750. In some embodiments, a collimation portion may be arranged tocollimate the first source light portion and second source light portionthat are output to the illumination grating. The beam separating portion1771 is arranged to input the source light 1731 and is configured tooutput a first source light portion 1731A and a second source lightportion 1731B to the illumination grating 1750, such that the firstsource light portion 1731A and the second source light portion 1731Bform beams that are spaced apart from one another along the measuringaxis direction 82. More specifically, a beam splitting surface 1775 ofthe beam splitter 1777 is configured to receive the source light 1731and reflect the first source light portion 1731A along a first beam pathand transmit the second source light portion 1731B toward the reflector1778. The reflector 1778 is configured to reflect the second sourcelight portion 1731B along a second beam path that is spaced apart fromthe first beam path. In the embodiment shown in FIG. 17, the beamsplitting surface 1775 and the reflector 1778 are parallel and aresurfaces of separate elements. Because they are parallel, the firstsource light portion 1731A and the second source light portion 1731B areparallel at a plane proximate to the illumination grating 1750. In someembodiments (e.g., the embodiment shown in FIG. 19), the beam splittingsurface 1775 and the reflector 1778 may be surfaces of the same beamsplitting element. In the embodiment shown in FIG. 17, the first andsecond beam paths are approximately perpendicular to the illuminationgrating. The illumination grating 1750 is configured to diffract thefirst and second source light portions 1731A and 1731B across anoperating gap to the scale track 1715 such that only two orders ofdiffracted light (i.e., a + first order diffracted light portion 1731A′from the first source light portion 1731A and a − first order diffractedlight portion 1731B′ from the second source light portion 1731B in theembodiment shown in FIG. 17) overlap within an imaged region IR at aplane coinciding with the scale track 1715, and provide the illuminationfringe pattern IFP in the imaged region IR. In contrast with theembodiments shown in FIG. 15 and FIG. 16, the illumination portion 1760outputs zero order light portions 1731ZA′ and 1731ZB′ which reach thescale track 1715. However, the zero order light portions 1731ZA′ and1731ZB′ fall completely outside the imaged region IR that is imaged tothe detector configuration 1725. Therefore, despite the fact that theyare not blocked, the zero order light portions 1731ZA′ and 1731ZB′ donot contribute unwanted zero order light to the operational signalsassociated with the illumination fringe pattern IFP at the imaged regionof the scale track 1715 that is imaged by the imaging configuration 1780to the detector configuration 1725.

In the embodiment shown in FIG. 17, the beam separating portion 1771 mayadditionally comprise an optional dual beam aperture element 1772. Thedual beam aperture element 1772 comprises two apertures configured totransmit the first source light portion 1731A and the second sourcelight portion 1731B while the dual beam aperture element 1772 reducesunwanted stray light, and may also refine the location and spacing ofthe first source light portion 1731A and the second source light portion1731B that reach the scale track 1715.

As shown in FIG. 17, the imaged region IR has a dimension D along themeasuring axis direction, the first source light portion 1731A and thesecond source light portion 1731B are spaced apart from one anotheralong the measuring axis direction 82 at the plane proximate to theillumination grating 1750 by a separation distance B, and the separationdistance B is equal to or greater than the imaged region dimension D.The first source light portion 1731A has a width W1 and the secondsource light portion 1731B has a width W2 at the plane proximate to theillumination grating 1750. Both W1 and W2 are greater than the imagedregion dimension D. This allows the imaged region to be filled with aspatially modulated illumination fringe pattern according to previouslydescribed principles, while the zero order light portions 1732ZA and1732ZB are separated by a sufficient distance at a plane proximate tothe scale track 1715, such that they may not overlap the +1 orderportion 1732A and the −1 order portion 1732B and, more importantly, falloutside of the imaged region.

In the embodiment shown in FIG. 17, the first source light portion 1731Aand the second source light portion 1731B are parallel at a planeproximate to the illumination grating. It should be appreciated that insome embodiments, the two are not parallel, but a gap distance Betweenthe illumination grating 1750 and the scale track 1715 and/or periods ofthe illumination grating 1750 or gratings of the scale track 1715 may beadjusted accordingly to obtain a desired fringe period of theinterference fringe pattern IFP imaged to the detector configuration1725.

FIG. 18 is a drawing of an illumination portion 1860 which may be usedin an encoder configuration 1800 according to the principles disclosedherein. The components and operating principles of the encoderconfiguration 1800 are approximately similar to those of the encoderconfiguration 1700 of FIG. 17, and may generally be understood byanalogy. For example, 18XX series numbers in FIG. 18 that have the same“XX” suffix as 17XX series numbers in FIG. 17 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below.

In the embodiment shown in FIG. 18, a light source 1830 is configured tooutput source light 1831 to a collimating lens 1840 which is configuredto collimate the source light 1831. The illumination portion 1860comprises a beam separation portion 1871 which consists of an apertureelement 1872 comprising an open dual aperture portion 1872 op. Theaperture 1872 element is configured to separate a first source lightportion 1831A and a second source light portion 1831B of the sourcelight 1831 and output them to the illumination grating 1850, such thatthey form beams that are spaced apart from one another along themeasuring axis direction 82 according to previously outlined principles.In particular, they are configured such that only two orders ofdiffracted light (i.e., a +1 order portion 1831A′ and a −1 order portion1831W) overlap within the imaged region IR at a plane coinciding withthe scale track 1815. The zero order light portions 1832ZA and 1832ZB donot contribute unwanted zero order light to the operational signalsassociated with the illumination fringe pattern IFP at the imaged regionof the scale track 1815.

FIG. 19 is a drawing of an illumination portion 1960 which may be usedin an encoder configuration 1900 according to the principles disclosedherein. The components and operating principles of the encoderconfiguration 1900 are approximately similar to those of the encoderconfiguration 1700 of FIG. 17, and may generally be understood byanalogy. For example, 19XX series numbers in FIG. 19 that have the same“XX” suffix as 17XX series numbers in FIG. 17 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below.

In the embodiment shown in FIG. 19, a structured illumination generatingportion 1970 comprises a beam separation portion 1971 which comprises ashear plate 1977. The shear plate 1977 is configured to reflect a firstsource light portion 1931A at a first region of a first surface 1977Aand to transmit a second source light portion 1931B that is reflectedfrom a second surface 1977B and output at a second region of the firstsurface 1977A. An illumination grating 1950 is configured to input thefirst source light portion 1931A and the second source light portion1931B such that the first source light portion 1931A and the secondsource light portion 1931B form beams that are spaced apart from oneanother along the measuring axis direction 82 according to previouslyoutlined principles. Reflectivity values of the first surface 1977A andthe second surface 1977B may be adjusted to give approximately equalintensities in the first source light portion 1931A and the secondsource light portion 1931B. For example, the first surface 1977A mayhave approximately 25% reflectivity and the second surface 1977B mayhave approximately 100% reflectivity. In another exemplary embodiment,the first surface 1977A may have 50% reflectivity, the second surface1977B may have 100% reflectivity, and a front surface 1977C may have 0%reflectivity where the second source light portion 1931B exits the shearplate 1977. The beam-separating portion 1971 is configured such thatonly two orders of diffracted light (i.e., a + first order portion1931A′ and a − first order portion 1931W) overlap within the imagedregion IR at a plane coinciding with the scale track 1915. The zeroorder light portions 1931ZA′ and 1931ZB′ do not contribute unwanted zeroorder light to the operational signals associated with the illuminationfringe pattern IFP at the imaged region of the scale track 1915.

FIG. 20 is a drawing of an illumination portion 2060 which may be usedin an encoder configuration 1900 according to the principles disclosedherein. The components and operating principles of the encoderconfiguration 1900 are approximately similar to those of the encoderconfiguration 1900 of FIG. 19, and may generally be understood byanalogy. For example, 20XX series numbers in FIG. 20 that have the same“XX” suffix as 19XX series numbers in FIG. 19 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below.

In the embodiment shown in FIG. 20, the illumination portion 2060comprises beam separation portion 2071 which comprises a first beamdirecting element 2077 and a second beam directing element 2078. Thefirst beam directing element 2077 is configured to reflect a firstsource light portion 2031A from a first surface 2077A and to transmit asecond source light portion 2031B that is reflected from a secondsurface 2077B and output to the second beam directing element 2078. Thesecond beam directing element 2078 is configured to reflect the secondsource light portion 2031B from a surface 2078A to the illuminationgrating 2050 and to reflect the first source light portion 2031A from asurface 2078C to the illumination grating 2050. In some embodiments, thefirst beam directing element 2077 is a shearing plate. In someembodiments, the second beam directing element 2078 is a shearing plate.In some embodiments, the surface 2078A comprises a reflective coating,which is useful in embodiments where the beam directing element 2078 isa shearing plate because this avoids an additional source light portionbeing transmitted through the surface 2078A and thus prevents the secondsource light portion 2031B from splitting into two source lightportions. In some embodiments, the second beam directing element 2078 isconfigured to transmit the first source light portion 2031A through anantireflective surface 2078B. In some embodiments, the surface 2078A andthe antireflective surface 2078B may be configured according to similarcombinations of reflectivity values as those described with respect tothe first surface 1977A and the second surface 1977B in FIG. 19. Theantireflective surface 2078B is useful when the beam directing element2078 is a shearing plate, as this avoids an additional source lightportion being reflected from the beam directing element 2078 thusprevents the first source light portion 2031A from splitting into twosource light portions. In lieu of a 100% reflective coating on theantireflective surface 2078B, an aperture similar to the dual beamaperture element 1772 of FIG. 17 may be used to block unwanted lightfrom a split light portion. The illumination portion 2060 isadvantageous in that the first source light portion 2031A and the secondsource light portion 2031B have equal optical pathlengths and wavelengthdependence.

In some embodiments, the second beam directing element 2078 may comprisethe reflective surface 2078A and a compensation prism 2078D (shown indotted lines). In such embodiments, the reflective surface 2078A may bea minor and the compensation may be configured such that the firstsource light portion 2031A and the second source light portion 2031Bhave equal optical pathlengths and wavelength dependence.

FIG. 21 is a drawing of an illumination portion 2160 which may be usedin an encoder configuration 2100 according to the principles disclosedherein. The components and operating principles of the encoderconfiguration 2100 are approximately similar to those of the encoderconfiguration 1700 of FIG. 17, and may generally be understood byanalogy. For example, 21XX series numbers in FIG. 21 that have the same“XX” suffix as 17XX series numbers in FIG. 17 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below.

In the embodiment shown in FIG. 21, the illumination portion 2160comprises a beam separation portion 2171 which comprises a first grating2190, a second grating 2191, a blocking element 2192, and an apertureelement 2193. The first grating 2190 is configured to split the sourcelight 2131 into a first source light portion 2131A and a second sourcelight portion 2131B which are +1 and −1 orders (or higher symmetricallymatched orders) of light diffracted from the first grating 2190. Theblocking element 2192 is configured to block any zero order componentsfrom the first grating 2190. The second grating 2191 (which comprisesthe same period as first grating 2190 in some embodiments) is configuredto receive the first source light portion 2131A and the second sourcelight portion 2131B and output a first parallel collimated light portion2131A′ and a second parallel collimated light portion 2131W which areparallel to one another. The aperture element 2193 is configured toreceive the first parallel collimated light portion 2131A′ and thesecond parallel collimated light portion 2131W and transmit them to theillumination grating 2150 while filtering out any additional orders oflight diffracted from the second grating 2191. In some embodiments, forthe highest efficiency, the first grating 2190 and the second grating2191 may be phase gratings.

FIG. 22 is a drawing of an illumination portion 2260 which may be usedin a reflective encoder configuration 2200 according to the principlesdisclosed herein. The components and operating principles of the encoderconfiguration 200 are approximately similar to those of the encoderconfiguration 2100 of FIG. 21, and may generally be understood byanalogy. For example, 22XX series numbers in FIG. 22 that have the same“XX” suffix as 21XX series numbers in FIG. 21 may designate similar oridentical elements, which may function similarly, except as otherwisedescribed or implied below.

The illumination portion 2260 incorporates many of the same elements ofthe illumination portion 2160 shown in FIG. 21 in a compact opticalarrangement utilizing reflective rather than transmissive gratingelements.

In the embodiment shown in FIG. 22, the illumination portion 2260comprises a light source 2230, a collimating lens 2240, and a structuredillumination generating portion 2270 comprising a beam separationportion 2271 which comprises a first grating 2290, a second grating2291, a blocking element 2292, an aperture element 2293, and a reflector2241. The light source 2230 is configured to output source light 2231 tothe collimating lens 2240. The collimating lens 2240 is configured tocollimate the source light 2231 and output it to the reflector 2241. Thereflector 2241 is configured to reflect the source light 2231 to thefirst grating 2290. The first grating 2290 is configured to split thesource light 2231 into a first source light portion 2231A and a secondsource light portion 2231B which are +1 and −1 orders (or highersymmetrically matched orders) of light diffracted from the first grating2290. The blocking element 2292 is configured to block any zero ordercomponents from the first grating 2290. The second grating 2291 isconfigured to receive the first source light portion 2231A and thesecond source light portion 2231B and output a first parallel collimatedlight portion 2231AP and a second parallel collimated light portion2231BP which are parallel to one another. The aperture element 2293 isconfigured to receive the first parallel collimated light portion 2231APand the second parallel collimated light portion 2231BP and transmitthem to the illumination grating 2250 while filtering out any additionalorders of light diffracted from the second grating 2291.

The imaging configuration 2280 comprises a first lens 2281, an aperture2282, a second lens 2283, and a reflector 2240. The reflector 2241 isconfigured to reflect scale light 2232 to the imaging configuration2280.

While various embodiments have been illustrated and described, numerousvariations in the illustrated and described arrangements of features andsequences of operations will be apparent to one skilled in the art basedon this disclosure. Thus, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An illumination portionfor an optical encoder which comprises a scale track extending along ameasuring axis direction, an imaging configuration, and a detectorconfiguration, the illumination portion comprising: a light sourceconfigured to output source light having a wavelength λ; and astructured illumination-generating portion configured to input thesource light and provide structured illumination, wherein the structuredillumination comprises an illumination fringe pattern orientedtransversely to the measuring axis direction which is input to the scaletrack, wherein: the scale track is configured to spatially modulate theinput illumination fringe pattern and output scale light comprisingspatially modulated image light; the detector configuration and imagingconfiguration are configured such that only scale light arising from animaged region of the scale track is imaged to the detectorconfiguration; the structured illumination-generating portion comprisesa beam-separating portion and an illumination grating; thebeam-separating portion is arranged to input the source light and isconfigured to output a first source light portion and a second sourcelight portion to the illumination grating, such that the first andsecond source light portions form beams that are spaced apart from oneanother along the measuring axis direction at a plane proximate to theillumination grating; and the illumination grating is configured todiffract the first and second source light portions across an operatinggap to the scale track such that only two orders of diffracted lightoverlap within the imaged region at a plane coinciding with the scaletrack, and provide the illumination fringe pattern in the imaged region.2. The illumination portion of claim 1, wherein: the imaged region has adimension D along the measuring axis direction, the first source lightportion and the second source light portion are spaced apart from oneanother along the measuring axis direction at the plane proximate to theillumination grating by a separation distance B, and the separationdistance B is equal to or greater than the imaged region dimension D. 3.The illumination portion of claim 2, wherein the first source lightportion and the second source light portion each have a dimension alongthe measuring axis direction, at the plane proximate to the illuminationgrating, that is greater than the imaged region dimension D.
 4. Theillumination portion of claim 1, comprising a collimation portionarranged to collimate the source light that is input to thebeam-separating portion.
 5. The illumination portion of claim 1,comprising a collimation portion arranged to collimate the first sourcelight portion and second source light portion that are output to theillumination grating.
 6. The illumination portion of claim 1, whereinthe two orders of diffracted light that overlap within the imaged regionat a plane coinciding with the scale track are a + first orderdiffracted light from the first source light portion and a − first orderdiffracted light from the second source light portion.
 7. Theillumination portion of claim 1, wherein: the beam-separating portioncomprises a first beam splitting surface and a reflective surface; thebeam splitting surface is configured to receive the source light sourceand reflect the first source light portions along a first beam path andtransmit the second source light portion toward the reflective surface;and the reflective surface is configured to reflect the second sourcelight portion along a second beam path that is spaced apart from firstbeam path.
 8. The illumination portion of claim 7, wherein the beamsplitting surface and the reflective surface are parallel.
 9. Theillumination portion of claim 7, wherein the beam splitting surface andthe reflective surface are surfaces of separate elements.
 10. Theillumination portion of claim 7, wherein the beam splitting surface andthe reflective surface are surfaces of the same beam splitting element.11. The illumination portion of claim 10, wherein the beam splittingelement is a shearing plate.
 12. The illumination portion of claim 7,wherein: the first and second beam paths are approximately perpendicularto the illumination grating.